In Microsoft Word, uncapable is marked as wrong. It sounded pretty alright to me, thus, I checked it up on the Web and found that many dictionaries do not have "uncapable" in their entries, but dictionaries such as Dictionary.com and Merriam-Webster list it to be a synonym of incapable. My question is: is uncapable considered a "proper" word?

I've heard both frequently, but only use "incapable" myself. I dare say that someone will invent a (spurious) rule for the difference, one day, based on his personal preferences. That's how it usually works.
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Mark WallaceJul 14 '11 at 12:51

Which has always puzzled me - who comes up with all these words?
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HahazJul 14 '11 at 13:30

Usually, they're mis-association of things that are common in other words. A great example is "inflammable" (= can catch fire), where large numbers of people assumed that the "in" was a prefix of negation, and used it to mean fireproof. The thing is: if enough people get it wrong, there's a chance that they're right to, and the word needed improvement. (That doesn't stop me hissing and spitting when it happens, though.)
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Mark WallaceJul 14 '11 at 13:43

4 Answers
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If a dictionary has it, that dictionary is just trying to be so comprehensive as to include any word ever. However, incapable is the proper and original form, and furthermore, everyone uses it. I have never seen uncapable in use.

The rule of thumb to go by is: If you're choosing between variants of a word, pick the variant most widely used and understood. So when you have to choose, choose incapable.

At that time, when picking the latinate prefix to indicate negation, whoever coined the word went with "in," not "un." A lot of words follow the pattern, e.g., "incredible," "inconceivable," which also date from the same period.

Once that form became set and fixed, there was no need for a version with "un-." I would flag "uncapable" as an error in a text and correct it, unless the term was being used for a special purpose, which could happen in an academic text.

I came to this site because I ran across the word uncapable in the writings of Mother Theresa. She speaks of God talking to her about becoming a sister of the poor. "You are I know the most uncapable person, weak and sinful, but just because you are that I want to use you, For My Glory! Wilt thou refuse?" Unable, unsuited, unskilled, unfit, unqualified are all synonyms of incapable. But somehow this non-word, uncapable, has a sense of a void or emptiness waiting to be filled, rather than the feeling of incompetence that the word incapable gives.

My understanding is that the "un-" prefix indicates the complete absence of something. For example, "uncapped" Internet access means that there is no cap; the cap is completely absent. Whilst the same argument could then be applied to "uncapable" (utterly lacking capability), the concept is more subtle than simply indicating the "absence of capability". "Incapable" means "not capable" rather than the absent of capability.

Un- and in- mean the exact same thing. In- is Latin, un- is Germanic (and a- is Greek). There is no such distinction between ‘not X’ and ‘complete absence of X’.
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Janus Bahs JacquetMar 10 '14 at 22:58